This invention generally relates to reconfigurable arrays of sensors (e.g., optical, thermal, pressure, ultrasonic). In particular, the invention relates to reconfigurable micromachined ultrasonic transducer (MUT) arrays. One specific application for MUTs is in medical diagnostic ultrasound imaging systems. Another specific example is for non-destructive evaluation (NDE) of materials, such as castings, forgings, or pipelines.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducers that are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. Such scanning comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is continuously refocused along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducers arranged in one or more rows and driven with separate voltages in transmit. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducers in a given row can be controlled to produce ultrasonic waves that combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused in a selected zone along the beam.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducers are summed so that the net signal is indicative of the ultrasound reflected from a single focal zone in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer. The time delays are adjusted with increasing depth of the returned signal to provide dynamic focusing on receive.
The quality or resolution of the image formed is partly a function of the number of transducers that respectively constitute the transmit and receive apertures of the transducer array. Accordingly, to achieve high image quality, a large number of transducers is desirable for both two- and three-dimensional imaging applications. The ultrasound transducers are typically located in a hand-held transducer probe that is connected by a flexible cable to an electronics unit that processes the transducer signals and generates ultrasound images. The transducer probe may carry both ultrasound transmit circuitry and ultrasound receive circuitry.
A reconfigurable ultrasound array is one that allows groups of subelements to be connected together dynamically so that the shape of the resulting element can be made to match the shape of the wave front. This can lead to improved performance and/or reduced channel count. Reconfigurability can be achieved using a switching network.
One advantage of MUTs is that they can be made using semiconductor fabrication processes, such as microfabrication processes grouped under the heading “micromachining”. As explained in U.S. Pat. No. 6,359,367:                Micromachining is the formation of microscopic structures using a combination or subset of (A) Patterning tools (generally lithography such as projection-aligners or wafer-steppers), and (B) Deposition tools such as PVD (physical vapor deposition), CVD (chemical vapor deposition), LPCVD (low-pressure chemical vapor deposition), PECVD (plasma chemical vapor deposition), and (C) Etching tools such as wet-chemical etching, plasma-etching, ion-milling, sputter-etching or laser-etching. Micromachining is typically performed on substrates or wafers made of silicon, glass, sapphire or ceramic. Such substrates or wafers are generally very flat and smooth and have lateral dimensions in inches. They are usually processed as groups in cassettes as they travel from process tool to process tool. Each substrate can advantageously (but not necessarily) incorporate numerous copies of the product. There are two generic types of micromachining . . . 1) Bulk micromachining wherein the wafer or substrate has large portions of its thickness sculptured, and 2) Surface micromachining wherein the sculpturing is generally limited to the surface, and particularly to thin deposited films on the surface. The micromachining definition used herein includes the use of conventional or known micromachinable materials including silicon, sapphire, glass materials of all types, polymers (such as polyimide), polysilicon, silicon nitride, silicon oxynitride, thin film metals such as aluminum alloys, copper alloys and tungsten, spin-on-glasses (SOGs), implantable or diffused dopants and grown films such as silicon oxides and nitrides.The same definition of micromachining is adopted herein. The systems resulting from such micromachining processes are typically referred to as “micromachined electro-mechanical systems (MEMS).        
The cMUTs are usually hexagonal-shaped structures that have a membrane stretched across them. This membrane is held close to the substrate surface by an applied bias voltage. By applying an oscillatory signal to the already biased cMUT, the membrane can be made to vibrate, thus allowing it to radiate acoustical energy. Likewise, when acoustic waves are incident on the membrane the resulting vibrations can be detected as voltage changes on the cMUT. A cMUT cell is the term used to describe a single one of these hexagonal “drum” structures. The cMUT cells can be very small structures. Typical cell dimensions are 25-50 microns from flat edge to flat edge on the hexagon. The dimensions of the cells are in many ways dictated by the designed acoustical response. It may not be possible to create larger cells that still perform well in terms of frequency response and sensitivity desired.
It is difficult to produce electronics that would allow individual control over such small cells. While in terms of the acoustical performance of the array as whole, the small cell size is excellent and leads to great flexibility, control is limited to larger structures. Grouping together multiple cells and connecting them electrically allows one to create a larger subelement, which can have the individual control while maintaining the desired acoustical response. So a subelement is a group of electrically connected cells that cannot be reconfigured. For the purpose of this disclosure, the subelement is the smallest independently controlled acoustical unit. One can form elements by connecting subelements together using a switching network. The elements can be reconfigured by changing the state of the switching network. However, subelements comprise connected cells that are not switchably disconnectable and thus cannot be reconfigured.
The current trend in ultrasound systems is toward smaller more portable devices. Perhaps one day a small palm-sized ultrasound system will replace the stethoscope as the physician's standard equipment. In order to realize such small ultrasound systems, reducing the number of beamforming channels is a must. The basic problem is to maintain adequate beamforming performance (resolution and contrast) while reducing the number of system channels. In order to maintain resolution, the aperture must not be reduced. This means for a standard array that either the number of channels must increase with increasing aperture size, or the spacing between them must increase. Increasing the step size in a uniform manner across the array results in grating lobes, which are not desirable.
All current ultrasound linear array probes have a constant pitch. Reducing channel count usually means giving up aperture or tolerating grating lobes. One method that reduces channel count without grating lobes is beamformer folding. For linear arrays the delays on either side of the beam center are symmetrical. This means that a reduction in channels by about one half can be achieved by allowing a multiplexing scheme that connects these symmetric elements to the same system channel. However, this multiplexing scheme can become very complicated for large numbers of system channels. Also, current multiplexing schemes cannot provide complete reconfigurability because they are limited to relatively large size elements at fixed locations. Another way that can be used to increase channel count is the use of synthetic aperture schemes. Here a larger aperture is built up over multiple transmits. This can lead to significant channel count reduction, but suffers from reduced signal-to-noise ratios and loss of frame rate due to the requirement of multiple firings. Motion can also introduce artifacts in the synthetic aperture images.
There is a need for a reduced channel count system that maintains aperture size and does not have grating lobes.